1. Field of the Invention
The present invention relates to a sublimation thermal transfer image recording material, and more particularly, to a sublimation thermal transfer image recording material having good half tone image reproduction ability in which a recorded color image has uniform hue regardless of its image density, as well as to an image recording material useful for multiple sublimation thermal transfer recording which can maintain good image qualities such as high image density and the good half tone images without difference of hue when the image recording material is repeatedly used multiple times of use for n-fold speed mode multiple sublimation thermal transfer recording material.
2. Discussion of the Related Art
Recently, the demand for full color recording has increased year by year. There have been known various full color recording methods including electrophotographic recording methods, ink jet recording methods and thermal transfer recording methods. Among these methods, thermal transfer recording methods are widely employed because of having the following advantages over the other recording methods:
(1) a full color image having good image qualities can be recorded relatively speedily without generating noise; and
(2) operation and maintenance of the recording apparatus are relatively easy.
The thermal transfer recording methods are broadly classified into a thermofusing thermal transfer recording method which transfers thermofusible ink onto a receiving material to form an image thereon and a sublimation thermal transfer recording method which transfers a thermo-diffusional dye (hereinafter referred to as a sublimable dye) onto a receiving material to form an image thereon. The sublimation thermal transfer recording method is superior to the thermofusing thermal transfer recording method because of having excellent half tone images caused by the transfer of a molecule of the thermo-diffusional dye. Therefore the sublimation thermal transfer recording method is a suitable method for full color recording.
In sublimation thermal transfer recording, a sublimation dye image can be obtained on a sublimation thermal transfer image receiving material (referred to as a receiving material) upon application of heat with a thermal printhead, laser or the like to the back side of a sublimation thermal transfer image recording material (referred to as a recording material) having an ink layer which contacts the receiving material and which includes a sublimable dye. The recording material includes a substrate and an ink layer which is formed on the substrate and includes a sublimable dye dispersed in a binder resin. A full color image can typically be obtained by appropriately transferring a yellow color dye, a magenta color dye, a cyan color dye and, if necessary, a black color dye onto a receiving material. The recording material may include a heat resistant layer on the back side thereof to make the recording material resistant to heat applied with thermal printheads. The receiving material includes a substrate and optionally an image receiving layer (referred to as a receiving layer) which is formed on the substrate. When heat is applied to the recording material, the sublimable dye diffuses into the receiving material or the receiving layer of the receiving material, so that an image is formed on the receiving material.
Sublimable dyes for use in the recording material have to have good diffusing ability under a heating condition in which a thermal printhead at high temperature (hundreds of degrees centigrade) contacts the recording material for a moment (several milliseconds) and have to have good color tone and good light resistance, to form an image having good image qualities such as high image density, good color tone reproducibility and good light resistance of the recorded image. In addition, the sublimable dyes have to be safe. There are few sublimable dyes having all of these properties. Therefore, a plurality of yellow dyes, magenta dyes, cyan dyes and if necessary, black dyes are indeed used for forming a full color image, although it is preferable that a full color image can simply be formed with one kind each of a yellow dye, a magenta dye and a cyan dye. For example, a magenta color recording material generally includes a red dye and a violet dye, a cyan color recording material includes a blue dye and a green dye and a black color recording material includes yellow, magenta and cyan dyes.
Recording materials are typically manufactured with a gravure coating method. When an image is recorded using a recording material which has a one layer type ink layer coated by a gravure coating method, the image tends to be uneven because the coated ink layer has unevenness corresponding to the form of cups of the gravure plate. Therefore, an ink layer is generally formed by coating twice a recording layer coating liquid including a resin and a sublimable dye which are dissolved or dispersed in a solvent (Japanese Laid-Open Patent Publication No. 63-302089) to form an even ink layer. Even in this case, when half tone images are recorded using a recording material having a recording layer including two or more sublimable dyes having different hues, the recorded half tone image tends to have different color tone depending on its image density. For example, when half tone images are recorded using a magenta colored recording material including a red sublimable dye and a violet sublimable dye, the recorded half tone image having relatively low image density has relatively violet-like (or reddish) magenta color compared to the half tone image having relatively high image density.
In addition, the sublimation thermal transfer recording method costs more than other methods, because:
(1) a sublimable dye is relatively expensive;
(2) yellow, magenta, cyan and if necessary, black color recording materials, each individually being of equal size to the recorded image, are needed to obtain a full color image; and
(3) used recording materials must be disposed of even though there may be large unused portions of the recording material.
To obviate these shortcomings, so-called multiple sublimation thermal transfer recording methods and recording materials therefor have been proposed. The multiple sublimation thermal transfer recording methods include an n-time (n is at least 2) mode multiple sublimation thermal transfer recording method and an n-fold (n is more than 1 and generally 5 to 20) speed mode multiple sublimation thermal transfer recording method. A recording material for the multiple sublimation thermal transfer recording methods is disclosed which can produce images having good image qualities such as high image density even in a large-n-time or a large n-fold multiple sublimation thermal transfer recording method. The recording material has two or more overlaid ink layers which are, for example, a dye-supplying layer which is formed on a substrate and which includes a sublimable dye dispersed in a resin, and a dye-transferring layer formed on the dye-supplying layer, wherein the dye releasing ability of the dye-supplying layer is larger than that of the dye-transferring layer.
In multiple sublimation thermal transfer recording, when a recording material including an overlaid ink layer in which at least the bottom ink layer includes two or more sublimable dyes is used, problems occur in which a color tone of the initially recorded image is different from that of the recorded image after the recording material is used n-times in the n-time multiple sublimation thermal recording or a color tone in a relatively light half tone image is different from that of a relatively dark half tone image.
Because of these reasons, a need exists for a sublimation thermal transfer recording material which can produce images having good image qualities, particularly produce half tone images having good evenness in color tone by a one-time or a multiple sublimation thermal transfer recording method.